(1) Field of the Invention
The present invention relates to a method and an apparatus for cutting patterns of printed wiring boards and a method and an apparatus for cleaning printed wiring boards, and more particularly, to methods and apparatuses of these kinds in which a laser beam is irradiated onto a printed wiring board so that the irradiated portion is evaporated, to thereby cut a pattern or clean the printed wiring board.
(2) Description of the Related Art
With a recent trend toward high packing density of electronic devices, there has been a demand for higher mounting density of printed wiring boards constituting electronic circuits, and attempts have been made to further the miniaturization of printed wiring and the lamination of wiring layers.
In such high-density wiring printed wiring boards, when the wiring pattern design is changed, a pattern on the surface of a printed wiring board of resin or an internal-layer pattern in the printed wiring board must be partly cut. Conventionally, wiring is cut with a drill, and also a method using a laser beam for cutting patterns has been proposed.
The method using a laser beam for cutting patterns of printed wiring boards will be briefly described. Generally, a YAG laser or excimer laser capable of pulsation is used, and a beam emitted from the laser is adjusted to the same width as a pattern to be cut, and is irradiated onto the pattern to cut same. In the case of a copper pattern, for example, about 0.1-.mu.m thickness can be removed by one pulse, and thus a copper pattern of about 50 .mu.m thick can be cut by about 500 pulses.
Also in the case of cutting an internal-layer pattern, a laser beam is irradiated to remove the resin on the internal-layer pattern, and then cut the internal-layer pattern.
In the conventional method using a laser beam for cutting patterns of printed wiring boards, however, when a pattern 451 formed on a surface of a printed wiring board PT is cut at a cutting spot 450, as shown in FIG. 1(a), molten matter A such as copper or resin scatters, as shown in FIG. 1(b), adheres to the surface of the printed wiring board PT. The adhering matter can cause defective insulation of the cut pattern 451.
In the case of cutting an internal-layer pattern, molten matter A such as copper or resin scatters, as shown in FIG. 2, adheres to inner walls 461 of a recess 460 which is formed by the removal of resin, thus also possibly causing defective insulation of the cut pattern.
Further, the internal-layer pattern cutting is associated with the following problems:
FIG. 3 is a sectional view of a printed wiring board and illustrates a conventional method using a laser beam for cutting an internal-layer pattern. Referring to the figure, when a first internal-layer pattern 412 of a printed wiring board 400 is to be cut by removing a region 412A, a laser beam 470B is irradiated onto the region 412A in the direction indicated by arrow 401. In this case, if the pattern width is not known beforehand, conventionally the first internal-layer pattern 412 is observed through a surface layer 410, the width of the laser beam 470B is adjusted to be equal to the observed pattern width W, and the laser beam 470B is irradiated onto the surface layer 410 from above so as to reach the first internal-layer pattern 412 to cut same.
However, when the width W of the first internal-layer pattern 412 is observed through the surface layer 410, it cannot be clearly recognized because of the presence of the surface layer 410 and a first resin layer 411. Thus, when the laser beam 470B is thereafter emitted with its width adjusted based on an inaccurate observed value of the pattern width W, the width of the beam 470B actually falling upon the first internal-layer pattern 412 may be greater than the pattern width W, as illustrated.
Further, the reflectivity of the laser beam 470B with respect to a metal (e.g. copper or solder) forming the internal-layer pattern greatly differs from that with respect to a resin (e.g., glass epoxy resin or polyimide resin) forming layers sandwiching the internal-layer pattern. Accordingly, when the laser beam 470B is irradiated, damage to the resin is several times heavier than that to the metal.
Thus, if the width of the laser beam 470B is greater than the pattern width W, the extra laser beam can easily melt a second resin layer 413 below the first internal-layer pattern 412, possibly cutting a second internal-layer pattern 414 apart. When cutting an internal-layer pattern by using a laser beam, therefore, it is difficult to properly cut the target internal-layer pattern alone.
Meanwhile, chlorofluorocarbons or trichloroethane has conventionally been used for cleaning printed wiring boards. To protect the ozonosphere, however, the use of the above mentioned fluid is being prohibited, and an alternative highly effective method for cleaning printed boards is demanded.
As an alternative to the method using chlorofluorocarbons or trichloroethane for cleaning printed boards, a non-washing method and a method using a substitute solvent are conventionally used.
In the non-washing method, printed boards are cleaned with low-active flux plus N.sub.2 reflow, and in the cleaning method using a substitute solvent, printed boards are cleaned with water.
These conventional methods, however, have the following drawbacks:
In the non-washing method, solder balls cannot be removed, and defective pin contact or corrosion of patterns are caused by the residual of the flux.
In the cleaning method using water as the substitute solvent, rust is likely to gather, and an equipment for treating the water used for the cleaning, a finish drying equipment and the like are required, which leads to an increase of the initial cost and the running cost.